(1) Field of the Invention
The present invention relates to the formation of asymmetric membranes characterized by a dense top "skin" layer. Such membranes find applications in gas separation as well as in reverse osmosis and pervaporation operations.
(2) Description of the Related Art
Successful gas separation operations require highly efficient semipermeable membranes. The membranes must fulfill certain requirements with regard to their selectivity, flux, and their chemical, thermal and physical stability.
Among various types of membranes suitable for gas separation, the so-called asymmetric membranes have revealed themselves to be exceptionally effective.
An asymmetric membrane may be defined as an entity composed of an ultra thin dense skin over a thick porous structure of the same or different material.
Asymmetric membranes have been made of various materials which include cellulose acetate, polyamides and polyimides.
The techniques for making asymmetric membranes from cellulose acetate polymers are well known and are disclosed in U.S. Pat. Nos. 3,133,132 and 3,133,137, issued on May 12, 1964 to Loeb et al.; 3,497,072, issued Feb. 24, 1970 to Cannon; and 4,026,978, issued May 31, 1977 to Mungle et al. Cellulose acetate membranes find wide application in the purification of water. However, it has been found that asymmetric cellulose acetate membranes have not been used without problems in applications such as reverse osmosis due to lack of temperature, chemical and microorganism resistance and besides these membranes are subject to degradation at pH extremes.
Polyamides are also employed in the manufacture of asymmetric membranes such as is disclosed in U.S. Pat. No. 3,567,632, issued to Richter et al. on Mar. 2, 1971. However, these membranes suffer the disadvantage of extreme sensitivity to degradation by trace quantities of chlorine and other oxidizing agents present in some process feed streams. Moreover, these polymers cannot withstand elevated temperatures.
Attention therefore has been drawn to the formation of asymmetric membranes from materials other than cellulose-based polymers and polyamides to provide membranes characterized by stronger structural properties and increased chemical resistance.
Attention has been focused on polyimide polymers, particularly aromatic polyimide polymers, as materials deemed valuable in the production of filtration membranes, and along these lines U.S. Pat. Nos. 3,899,309 by Harvey H. Hoehn et al. and 4,690,873 by Hirashi Makino may be cited. These references, while teaching aromatic polyimide materials as useful in the manufacture of gas separation membranes, do not teach formation of asymmetric membranes. U.S. Pat. No. 4,690,873 discusses dense film formation using a high boiling, toxic solvent (p-chlorophenol).
The development of asymmetric membranes made of polyimide polymers has generally been heralded as successful in meeting the needs for gas separation membranes not fulfilled by prior art materials.
The inherently greater chemical, biological, and thermal stability of polyimides over the membrane materials used in the prior art makes them attractive candidates for use in reverse osmosis, pervaporation and gas separation applications. This was recognized by J. K. Beasley of E. I. DuPont de Nemours & Company in a paper presented at the Dec. 1977 meeting of IDEA in Tokyo, where he mentioned polyimides as more suitable for reverse osmosis applications particularly, rather than three hundred other candidate polymer compositions tested.
Asymmetric membranes have a porous structure, the average pore size being in the range of from 0.01 to 1 .mu.m, which has an extremely thin homogeneous layer on the upper side which is the actual membrane, while the porous substructure serves only as a support and has no influence on the separation characteristics. Since the active layer determining the flux of the asymmetric membranes is extremely thin (0.05 to 0.5 .mu.m) relatively high fluxes are possible.
Asymmetric membranes are usually prepared by a so-called precipitation or phase inversion reaction. For this purpose, the polymer used for preparing the membrane is dissolved in a suitable solvent, spread into a film, and precipitated in a non-solvent.
The following U.S. patents constitute documentation of the prior art relating to the manufacture of asymmetric semipermeable membranes of polyimide materials.
U.S. Pat. No. 3,925,211, Wilhelm Schumann et al., teaches asymmetric polyimide membrane preparation by a process comprising first preparing membranes having asymmetric structures from acid amides, capable of being converted to polyimides, according to the usual precipitation or phase inversion reaction and subsequently converting these acid amide membranes to polyimide membranes by a thermal or chemical ring closure reaction.
The following is the reaction scheme shown in this reference and in references teaching variations on the same theme of starting from a tetracarboxylic acid dianhydride and a diamine to form, via an acid amide, a polyimide. ##STR1## R and R' are aliphatic or aromatic groups and n is in each case selected to achieve a film forming prepolymer.
The polymers used for preparing the membrane are dissolved in a suitable solvent, spread into a film and precipitated in a non-solvent. No solvent evaporation takes place in the process taught in this reference and the area of applicability taught is that of ultrafiltration, i.e., filtration of molecular range particle sizes requiring pore sizes ranging from 0.002 to 0.1 micrometers.
U.S. Pat. No. 4,113,628 by Constance Wright Alegranti, teaches preparation of asymmetric polyimide membranes by a process comprising preparation of a polyamic acid precursor solution from diamines and tetracarboxylic acid dianhydrides, contacting said solution with a selected chemical cyclizing agent which results in the precipitation and cyclization of the polyamic acid to the polyimide. The polyimide is washed to remove solvents and no solvent evaporation step takes place. The membranes prepared according to the teachings of this reference are said to be useful in the reverse osmosis area of application, i.e., an area requiring pore sizes characterized by ionic range particle sizes ranging from 0 to 0.001 micrometers.
U.S. Pat. Nos. 4,440,643 and 4,485,056 by H. Makino et al teach asymmetric polyimide preparation also from a dope solution of a corresponding polyamic acid which is a precursor polymer of the polyimide, by coating one surface of a substrate of an aromatic polyimide with a solution containing an organic polar solvent to form a thin layer of a polyamic solution whose solvent is evaporated as the polyamic acid is cyclized to the polyimide. The area of applicability taught for these references is that of gas separation, i.e., an area requiring pore sizes in the 0-0.001 micrometer range.
U.S. Pat. No. 4,532,041 by Harry F. Shuey teaches an asymmetric polyimide membrane useful for reverse osmosis by a process comprising forming a film of the polyamic acid dope solution, contacting the film with a coagulating liquid, drying and heating to cyclize the polyamic acid of the corresponding polyimide.
U.S. Pat. No. 4,902,422 by Injo Pinnau et al. teaches a gas separation asymmetric polyimide membrane utilizing forced convection to induce phase separation, i.e., under carefully controlled conditions a moving stream is used across a static nascent membrane.
It should be also noted that polyimides can be made by other methods, for example by reaction of dianhydrides with other nitrogen-bearing polyfunctional compounds such as diisocyanates.
Finally attention is called to U.S. Pat. No. 4,908,134 by Bryce P. Anderson. This reference distinguishes itself from the previously discussed prior art in that instead of teaching asymmetric polyimide membrane preparation by phase inversion of a polyamic acid precursor, it teaches preparation of the polyimide membrane by using an already fully imidized polymer as the starting material. The polyimide material is dissolved in a solvent to which a pore forming agent is added, which is characterized by having at least about 50% more molar volume than the solvent or solvent mixture used, a solubility parameter equal to a greater than that of the solvent, a polar end, and substantial hydrocarbon character. The solution is cast onto a support and coagulated in an appropriate solvent such as water to displace the original dissolving solvent. The porous character membrane thus produced is claimed as useful in ultrafiltration applications, an operation requiring a pore size range of 20 to 2000 .ANG.. This membrane does not have a dense top "skin" layer.
The present invention concerns itself with this latter procedure of using fully imidized polymers as starting materials in the manufacture of asymmetric polyimide membranes; however it is directed to producing membranes useful for applications in the gas separation area. Such membranes can additionally be used in the reverse osmosis and pervaporation areas. These membranes are characterized by the presence of a dense top "skin" layer having a pore size which is substantially smaller than that used in ultrafiltration applications.
Details of the invention's process for producing asymmetric polyimide membranes will therefore be outlined as the description of invention proceeds.